Letter pubs.acs.org/journal/estlcu
Why Dissolved Organic Matter Enhances Photodegradation of Methylmercury Yun Qian, Xiangping Yin, Hui Lin, Balaji Rao, Scott C. Brooks, Liyuan Liang, and Baohua Gu* Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, United States S Supporting Information *
ABSTRACT: Methylmercury (MeHg) is known to degrade photochemically, but it remains unclear what roles naturally dissolved organic matter (DOM) and complexing organic ligands play in MeHg photodegradation. Here we investigate the rates and mechanisms of MeHg photodegradation using DOM with varying oxidation states and origins as well as organic ligands with known molecular structures. All DOM and organic ligands increased the rate of MeHg photodegradation under solar irradiation, but the first-order rate constants varied depending on the oxidation state of DOM and the type and concentration of the ligands. Reduced DOM photochemically degraded MeHg 3 times faster than oxidized DOM. Compounds containing both thiol and aromatic moieties within the same molecule (e.g., thiosalicylate and reduced DOM) increased MeHg photodegradation rates far more than those containing only aromatics or thiols (e.g., salicylate or glutathione, or their combinations). The mechanism is attributed in part to strong binding between MeHg and thiolates that resulted in direct energy transfer from excited triplet state of the aromatics to break the Hg−C bond in MeHg. Our results suggest that, among other factors, the synergistic effects of thiol and aromatics in DOM greatly enhance MeHg photodegradation.
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MeHg−DOM complexes16 may be involved. FernandézGoméz et al.6 proposed that the light spectrum dependent MeHg photolysis is due to a cocktail of different organic radicals and ROS that are present over a wide spectral range. The differences observed in these previous studies illustrate important gaps in our understanding of the roles of DOM in MeHg photodegradation. Thus, the primary objective of this study is to determine the rates and mechanisms of MeHg photodegradation influenced by DOM. Several organic ligands with known molecular structures were selected as DOM analogues in the study. We examined (1) the effects of two types of DOM (either aquatic or terrestrial) and oxidation states (either reduced or oxidized), (2) the chemical structure and functional groups (e.g., aromatics and thiols) of selected ligands, and (3) the roles of 1O2 and •OH radicals and the excited triplet state of DOM (3DOM*) in MeHg photodegradation. We hypothesized that thiolate and aromatic moieties in DOM work synergistically, which results in strong MeHg−DOM binding and direct energy transfer from 3DOM* to break the Hg−C bond in MeHg.
INTRODUCTION Monomethylmercury (MeHg) has been studied extensively with regard to its environmental fate and transformation because of its potent neurotoxicity and bioaccumulation in the food web.1−6 Both biotic and abiotic processes can break down MeHg,7−9 but photochemical degradation is considered a major sink for MeHg and is responsible for as much as 80% of demethylation in some freshwater lakes.10−13 Studies have shown that ultraviolet (UV) radiation under natural sunlight primarily drives the photolysis of MeHg in surface waters,5,6,14,15 but the reaction mechanism remains poorly known, especially regarding the roles of dissolved organic matter (DOM) and complexing organic ligands in MeHg photodegradation. There is currently a lack of consensus regarding how and if DOM is involved in the photolysis of MeHg.5,6,16,17 Several studies reported a positive correlation between MeHg photodegradation and DOM concentration,6,11,16,17 but others found no significant influence of DOM on photodegradation.5,12 For example, labile iron, not DOM, and reactive oxygen species (ROS, such as hydroxyl radical, •OH) were shown to be important in MeHg photodegradation.12 Other studies17 found that MeHg photodegradation was greatly enhanced by DOM and that singlet oxygen (1O2) radicals, not •OH, were responsible. However, recent studies by Black et al.5 and Tai et al.16 found that none of these parameters alone (1O2, •OH, thiols, DOM, and iron concentrations) is effective in MeHg photodegradation. These authors suggest that multiple pathways5 or direct photolysis of © 2014 American Chemical Society
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MATERIALS AND METHODS DOM Samples and Organic Ligands. Two DOM samples were used in this study; one was isolated from the Received: June 14, 2014 Accepted: September 2, 2014 Published: September 2, 2014 426
dx.doi.org/10.1021/ez500254z | Environ. Sci. Technol. Lett. 2014, 1, 426−431
Environmental Science & Technology Letters
Letter
To examine if 3DOM*, 1O2, or •OH radicals may be involved in MeHg photodegradation, experiments were performed in the presence of 10 mM isoprene (a triplet state quencher), 10 mM sodium azide (NaN3, a 1O2 quencher), or 0.01% (v/v) isopropyl alcohol (a •OH scavenger) as used in previous studies.5,16,17 Salicylate (2 μM), thiosalicylate (5 nM), and reduced EFPC-DOM (0.13 mM DOC) were used in these experiments, and all other experimental conditions were identical to those described above.
contaminated East Fork Poplar Creek (EFPC-DOM) in Oak Ridge, TN,18 and the other was a reference Elliot soil humic acid (E-SHA) from the International Humic Substances Society.19,20 They were used either in the oxidized or reduced state (by H2/Pd reduction), as described previously.20 Six organic ligands were used as DOM analogues,17,21−23 including thiosalicylate and 6-thioguanine (aromatic thiols), salicylate (aromatic carboxyl), cysteine and glutathione (nonaromatic thiols), and acetate (nonaromatic carboxyl) (see Figure S1 of the Supporting Information for the chemical structure and UV−visible absorbance spectra). To minimize the oxidation of thiol compounds, reagent solutions were prepared in deoxygenated water and samples handled in an anaerobic chamber.24 Photodegradation Experiments. MeHg reactant solutions were prepared in 100 mL volumetric flasks by adding stock DOM or ligand solutions and then the MeHg stock, which was prepared freshly for every batch of the experiment from a certified standard [5 μM, in 0.2% HCl and 0.5% (v/v) acetic acid] from Brooks Rand Laboratories (Seattle, WA). Unless otherwise specified, the final MeHg concentration was 50 pM (with 0.87 μM acetate), the ligand concentration was 2 μM, and the DOM concentration was ∼0.13 mM DOC (dissolved organic carbon). All experiments were performed under low-ionic strength conditions (∼1 mM KCl), typically observed in natural freshwater,25 and the pH was adjusted and maintained at 6.9 ± 0.1. For concentration-dependent studies, the final concentrations of thiosalicylate ranged from 0.1 nM to 2 μM and those of EFPC-DOM ranged from 0.01 to 0.13 mM DOC, equivalent to an estimated reactive thiol content of approximately 1−12.8 nM.18,19 The reactant solutions (3 mL each) were then pipetted into a series of 4 mL clear borosilicate glass vials (Wheaton Industries) with a light transmission efficiency of >80% from 300 to 800 nm (Figure S2 of the Supporting Information). Control samples were prepared in the same way either without organic ligands or with organic ligands but in the dark (using amber glass vials wrapped with aluminum foil). Photodegradation experiments were initiated by placing samples in a simulated solar chamber (Suntest XLS+) (see Figure S3 of the Supporting Information for the spectrum and additional information). The light irradiance was 40 W m−2 for UV (300−400 nm) and ∼373.6 W m−2 total (300−800 nm), typical midsummer conditions in eastern Tennessee (Figure S4a of the Supporting Information). 26,27 Under these conditions, similar rates of MeHg photodegradation were observed using either simulated or natural sunlight (Figure S4b of the Supporting Information). Two or three samples were analyzed for MeHg at selected times by modified EPA method 1630 (Supporting Information). Selected samples were also analyzed for total Hg (Supporting Information), and the recovery was usually within ±10%. The pseudo-first-order rate constant (kd) of MeHg photodegradation was determined using eq 1: ln[MeHg]t = ln[MeHg]0 − kdt
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RESULTS AND DISCUSSION We first examined whether DOM enhances MeHg photodegradation in the presence of either reduced or oxidized EFPC-DOM or E-SHA. In all cases, addition of DOM increased the rate of photodegradation of MeHg, and the MeHg concentration decreased exponentially over time (Figure 1a). Approximately 90% MeHg was degraded in a week with the reduced DOM, whereas only 50−60% of MeHg was degraded with the oxidized DOM. The kd‑ph values (±standard deviation) for reduced and oxidized EFPC-DOM were (4.3 ± 0.3) × 10−3 and (1.4 ± 0.1) × 10−3 m2 E−1, respectively, and were (3.2 ± 0.2) × 10−3 and (1.0 ± 0.1) × 10−3 m2 E−1 for reduced and oxidized E-SHA, respectively (Table S1 of the Supporting Information). These values were comparable to those reported for coastal seawater (3.2 × 10−3 m2 E−1) and for boreal lakes and wetlands (2−7.5 × 10−3 m2 E−1).5,6 Controls without DOM (but with a 0.87 μM acetate background) and dark controls with DOM exhibited negligible MeHg degradation (
